This invention pertains to distributed-feedback dye lasers, and in particular, to such lasers having adjustable mirrors for directing a pumping beam at a dye cell over a range of angles.
A distributed-feedback dye laser (DFDL) provides a narrow line width, coherent light source in which the refractive-index and gain modulations are induced by an interference pattern produced when a pair of pumping beams impinge a dye cell region from generally opposed, angled directions. The feedback required for laser operation results from Bragg scattering from the spatially periodic laser medium susceptibility induced by the interference of the two coherent pump beams. Typical DFDL systems are shown in an article entitled "Prism-Dye Laser" by Chandra, et al., Appl. Phys. Lett., Vol. 21, No. 4, Aug. 15, 1972, pp. 144-146, and in an article entitled "A Novel Pumping Arrangement for Tunable Single Picosecond Pulse Generation with a N.sub.2 Laser Pumped Distributed Feedback Dye Laser" by Bor, Optics Communications, Volume 29, No. 1, April 1979, pp. 103-108. Such systems normally have an index-of-refraction-matching prism disposed on the dye cell to facilitate directing the pumping beam into the dye cell. In such systems the dye laser wavelength is given by EQU .lambda..sub.L =(n.sub.s .lambda..sub.p)/(n.sub.p sin .theta.)
where .lambda..sub.p is the pumping wavelength, n.sub.s and n.sub.p are, respectively, the refractive indices of the dye solution at .lambda..sub.L and the prism at .lambda..sub.p, and the angle .theta. is the angle of incidence at the prism-dye interface. The wavelength is thus inversely proportional to the angle of incidence.
In such systems it is typical to either split a pumping beam and reflect it from a pair of tuning mirrors to the dye cell or to use a prism on the dye cell having a highly reflective face normal to the face of the dye cell with half of the impinging pumping beam being reflected from the face onto the other half of the beam which impinges the dye cell. Such systems are tuned by varying the angle of incidence of the pumping beam on the prism or, if there is no prism, on the dye cell. The resulting laser wavelength varies approximately 8 nm per degree of change of angle of incidence.
Heretofore, adjustment of the angle of incidence on the dye cell has included translating a tuning mirror along the path of the pumping beam incident on the mirror or rotating the mirror in place. Alternatively, the prism/dye-cell assembly is rotated. Such adjustment methods have very limited ranges of angle adjustment because the resulting two incident pumping beams only overlap completely at a single orientation of the mirror(s) relative to the dye cell. Small angular variation of either the dye cell or the tuning mirror shifts the incident pumping beams. Also, rotation of the dye cell causes a corresponding rotation of the output laser beam. For example, assume the length of the "footprint" of a beam impinging on a dye cell at an angle of 45.degree. from a mirror 10 cm away, is 1.60 cm in length. If the mirror is rotated in place one degree, the footprint shifts 0.12 cm. Thus, the amount of overlap is reduced to approximately 1.36 cm, or 85% of the original overlapped length. An angle shift of approximately 6.degree. results in no overlap at all. If the tuning mirror is simply translated along the path of the beam incident on the mirror, the reflected beam which is incident on the dye cell is moved correspondingly. Thus, there would be very little movement allowed of the tuning mirror (approximately 0.6 cm in the example above) before, again, there is no overlap between the two impinging pumping beams. It can be seen that such systems are extremely sensitive to movement of the tuning mirror.
Conventional DFDLs are also inefficient in the construction of the dye cell container relative to a laser beam generated in the dye cell. In particular, laser exit surfaces are constructed in planes disposed either perpendicular to the laser beam direction, or more commonly, at an angle oblique to the laser beam but which intersect a plane, parallel with the impinged region of the dye solution and containing the center line or axis of the laser beam, to define a line perpendicular to the laser beam. Although the oblique angle of such a surface relative to the laser beam reduces unwanted reflection back into the dye cell, it does not completely prevent it.
The present invention overcomes the noted disadvantages of conventional DFDLs. The invention includes means for adjusting the position of the tuning mirror along the incident pumping beam path such that the mirror directs the resulting reflected beam toward the dye cell region in a manner impinging substantially the same portion of the region for the positions of adjustment of the mirror along the path. This preferably includes changing the angular orientiation of the mirror simultaneously with translation of the mirror along the incident beam path. The preferred embodiment includes an adjustable equilateral-parallelogram configuration of links controllably pivotally coupled to the tuning mirror. In addition, means are provided for varying the direction of the pumping beam prior to reflection by the tuning mirror and thereby, for varying the direction of the beam reflected by the tuning mirror.
As another salient feature of the present invention, the dye cell surfaces through which the generated lasing beam passes are preferably oriented relative to the lasing beam at an oblique angle relative to the laser beam and perpendicularly relative to a plane both containing the laser beam and extending generally parallel with the dye cell surface which is impinged by the pumping beams.
These and additional features and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following detailed description of the preferred embodiment.